Per-provider proxy support for server-side fetch calls

Node.js native `fetch()` doesn't respect `HTTP_PROXY`/`HTTPS_PROXY`. We have a `proxyFetch` wrapper using undici, but currently only Tavily web search uses it. All other server-side providers (LLM, TTS, ASR, image, video) use raw `fetch()`, causing `fetch failed` errors for users who need a proxy.

Blanket proxying isn't ideal either — domestic providers (Seedance, Kling, Qwen, etc.) don't need it and may break if routed through an overseas proxy. A per-provider `proxy: true` flag in config would let each provider opt in.